Real-time medical ultrasound imaging has played an increasingly important role in the diagnosis and treatment of disease. Ultrasound imaging is used for routine diagnostic procedures in obstetrics, gynecology, cardiology, and gastroenterology. The vast majority of ultrasound systems in use today provide two-dimensional (2D) cross-sections of the anatomy. While other imaging modalities such as magnetic resonance imaging and x-ray computed tomography have provided three-dimensional (3D) images since their inception, only recently have 3D ultrasound imaging systems become commercially available. These systems have the potential to revolutionize medical imaging by providing 3D visualization of the anatomy and blood flow in real-time.
Conventional hardware and methods used for 2D ultrasound systems do not scale well to achieve similar 3D imaging systems. Modern 2D ultrasound scanners use a long 1D-transducer array having roughly 128 elements. Transducer array length and number of elements used is chosen based on several design parameters, including operating frequency and desired lateral resolution. An equivalent 3D imaging system capable of achieving similar resolutions in both azimuth and elevation would require a square 2D transducer array with 128 elements per side, or a total of 16,384 elements. A first challenge one faces when implementing such a system is fabricating the transducer array with reasonable yields.
A second challenge caused by a large channel count for a 3D ultrasound system is implementing the highly parallel front-end electronics required. Front-end hardware has become one of the most space- and power-consuming parts of a typical ultrasound imaging system. This is especially true since the advent of digital beamforming to vary transmit and receive directions and focal lengths, which has greatly reduced back-end hardware requirements. Unfortunately, the analog nature of the front-end hardware has not experienced an equal reduction in cost and size. High-end commercial ultrasound machines still house the analog and mixed-signal, front-end electronics within a base unit, requiring costly and bulky probe cables that contain dedicated coaxial transmission lines for each transducer element.
Modern 2D imaging systems require this complex set of front-end electronics because they typically use conventional full phased array (FPA) imaging, which requires that all array elements be simultaneously active during transmit and receive. See, for example, A. Macovski, “Medical Imaging Systems” (Prentice Hall, Englewood Cliffs, N.J., 1983). As shown in FIG. 1, in an FPA imaging system 100, for every transducer element that is active for a given firing event (110, 112, 114, 116, 118, 120, 122, 124), an independent front-end transmit (126, 128, 130, 132, 134, 136, 138, 140) and receive (142, 144, 146, 148, 150, 152, 154, 156) electronics channel must perform pulse generation, transmit/receive switching, amplification, filtering, time-gain compensation and digital-to-analog conversion in parallel. These electronics are the primary contributor to the bulk, cost, and power consumption of a typical ultrasound imaging system. In addition to high front-end hardware complexity, the large number of received signals required to form each beam causes a significant increase in transmit beamformer 158 and receive beamformer 160 complexity. The implementation of precision delay lines for beam steering also places a large burden on the beamforming hardware. Using all elements for transmit and receive results in the best image quality, improves signal-to-noise ratio (SNR) by maximizing total transmitted signal power, improves overall sensitivity for receiving echo signals, and has a very high frame rate since only one transmission or firing is required for each transmit direction. While electronic components continue to become smaller, faster, and cheaper, it is still not feasible to implement a full set of channels required for a 2D transducer array for 3D ultrasonic imaging.
The need to reduce the number of channels in a 3D imaging system has been recognized for some time, and several approaches have been presented in the art. One approach is the use of sparse arrays, which define a fixed subset of active elements that span a full aperture of the array. Different methods for choosing active elements include random and periodic distributions. Other array geometries intended to reduce the channel count include boundary arrays and a Mill's cross array. While these methods successfully reduce the channel count of the system, they suffer from high side lobes (and thus poor contrast resolution) and low signal-to-noise ratio (SNR).
Alternative beamforming methods have also been suggested. As shown in FIG. 2, classical synthetic aperture (CSA) imaging techniques employing a single channel (or a few neighboring channels) for transmit and receive minimize the hardware complexity. In a CSA imaging system 200, a transmit/receive controller 210 provides drive signals to an active element 216 via front-end transmit electronics 212 and a multiplexer 214 and receives received signals via the multiplexer 214 and front-end receive electronics 218. See, for example, U.S. Pat. No. 4,839,652. CSA was first used with linear arrays with reconstruction in the spatial domain, but has since been modified for use with circular arrays and frequency-domain reconstruction methods have also been developed. For a standard linear array method, a single processing channel is time-multiplexed across all transducer elements. Since only a single element is used for both transmit and receive, the complexity of the front-end electronics is kept to an absolute minimum; however, transmitted power and receive sensitivity are minimal and lead to low SNR. Each image pixel is reconstructed using all echo scans; time separation between scans leads to tissue motion artifacts. When used to construct images from an array with an element pitch equal to half of a minimum wavelength, CSA also suffers from high grating lobes. To avoid the grating lobes, element pitch is typically chosen to be a quarter of the minimum wavelength, but at the expense of reducing the physical aperture (and the related lateral resolution) by a factor of two for the same element count. CSA also requires multiple transmissions for each transmit direction and adversely impacts the frame rate.
In synthetic phased array (SPA) imaging with a single active element per data acquisition step, each image pixel is formed by coherent summation of signal contributions from every transmit/receive element combination. (SPA imaging is also shown in FIG. 2.) See, for example, U.S. Pat. Nos. 4,586,135 and 5,465,722. SPA processing produces images with comparable resolution and SNR to the FPA images with lower front-end complexity. However, there is a significant increase in the number of transmissions for each image frame with the usual adverse impact on the frame rate. In addition, the technique is limited by a limited transmit/receive power from a single active channel, which necessitates especially low electronic noise front-end electronics.
Array imaging techniques have continued to strike compromises between CSA and FPA, aiming to improve the SNR of CSA methods and reducing the number of channels required for FPA imaging. An early proposal for reducing the number of active channels in phased array imaging systems did so by transmitting on a single central portion of the array and receiving on a number of overlapping or adjacent subarrays. See, for example, U.S. Pat. No. 4,553,437 and L. F. Nock et al., “Synthetic Receive Aperture Imaging with Phase Correction for Motion and for Tissue Inhomogeneities. I: Basic Principle,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 39, pp. 489-95 (1992). Later developments improved the frame rate of subarray imaging by acquiring a subset of the beam lines and interpolating the others. See, for example, M. Karaman, “Ultrasonic Array Imaging Based on Spatial Interpolation,” 3rd IEEE International Conference on Image Processing, pp. 745-748 (1996) and U.S. Pat. No. 5,940,123. These methods, however, use 1D lateral interpolation filters and thus only produce successful results for relatively narrowband imaging. Recent proposals include transmitting from multiple elements to emulate a more powerful transmit element in SPA imaging, although a correction for motion and phase aberration would be required. A similar method proposes transmitting from five virtual elements and using the full aperture in receive in order to achieve the higher frame rates needed for 3D imaging with a 2D transducer array.
Real-time ultrasound imaging systems represent a tradeoff between front-end electronic complexity, image quality, SNR and frame rate. The proposals in the prior art do not successfully combine the advantages of CSA imaging in terms of reduced front-end complexity with the high quality image, high SNR and high frame rate associated with FPA imaging. Accordingly, there remains a need for a novel imaging system that combines the advantages of FPA and CSA imaging systems.